Protective coating for metals

ABSTRACT

A process for protecting metal from the deleterious effects of interaction with atmospheres surrounding said metal which comprises forming on the surface of said metal a continuous film of green chromium oxide and silica, the green chromium oxide portion of said film being directly bonded to the underlying base metal and coacting with the silica to provide a film impervious to gas at temperatures of about 500*C. to about 1300*C. The invention contemplates the product so protected and a composition of matter comprising chromium metal powder dispersed in an alkali-stabilized silica sol particularly adapted to be used in forming the green chromium oxide-silica film on metals.

United States Patent 1191 1111 3,861,938 Jackson Jan. 21, 1975 [54] PROTECTIVE COATING FOR METALS 3,298,936 1/1967 Michael 117 70 c 3,356,515 12/1967 McClothlin... 106/290 [75] Invenmr- Raymmd Penmye' Jacks, 3,565,663 2/1971 Costello et al.... 117/1351 Suffer, 3,677,796 7/1972 Girard etal. 117/705 [73] Assignee: The International Nickel Company Inc., New York, NY. Primary ExaminerCharles E. Van Horn A'11E-'hlW.1l 2] Fi ed: Nov. 20, 19 2 SSZS an xammer MIC ae Ba [21] Appl. No.: 307,862 [57] ABSTRACT Related [1.8. Application D ta A process for protecting metal from the deleterious [63] continuatiommpan of Ser 234,311 March 13 effects of interaction with atmospheres surrounding 1972. said metal which comprises forming on the surface of said metal a continuous film of green chromium oxide [52] US. Cl 117/31, 106/14, 117/70 c, and Silica, the green Chromium oxide portion of said 117/70 5 117/127 117/13'3Tf film being directly bonded to the underlying base 1 17 1 9 A, 143 2 metal and coacting with the silica to provide a film im- [51 Int. Cl CZ3C 3/00 porvious to gas at tomporawros of about 500C to [58] Field of Search 117/135.1, 70 c, 70 R, about 1300O The invention contemplates tho P 1 7/70 SR, 71 M, 1 9 A, 127 53 17 22 33 uct so protected and a composition of matter compris- 31; 106/74, 84, 290, 1, 14; 148/62 ing chromium metal powder dispersed in an alkalistabilized silica sol particularly adapted to be used in 5 References Cited forming the green chromium oxide-silica film on met- UNITED STATES PATENTS 2,978,361 4/1961 Seidl et a1. 117/169 A 30 Claims, N0 Drawings 3,175,920 3/1965 Michael 117/70 D PROTECTIVE COATING FOR METALS This application is a continuation-in-part of US. application Ser. No. 234,311, filed Mar. 13, 1972.

The present invention is concerned with the protection of metal surfaces from interaction with the atmosphere surrounding them, especially at temperatures of hot working.

It is well known that certain metals, particularly iron and low alloy and tool steels, are subject to high temperature oxidation when heated above about 1000F. (about 540C.). For example, when heated in air up to 2350F. (about l290C.), preliminary to hot rolling or forging, a low alloy steel can lose up to about 4% of its metal by scaling during the soaking period necessary to bring a billet up to working temperature. In an oxidizing atmosphere, a tool steel or a carbon-containing nickel anode material can lose a substantial percentage of the carbon content in the skin portion of the item being heated. Skin decarburization requires deskinning of the item which has been heated, or, alternatively, a separate operation of recarburizing. Under different circumstances where a metal such as iron is in contact at high temperatures with a carbon-containing atmosphere, it is possible to carburize the metal surface when carburization is not desired.

The problem with scaling of iron-base alloys is not limited to the scale loss mentioned hereinbefore. With some alloys, notably steel containing about 9% nickel useful for cryogenic and other purposes, scale formation tends to dilute the alloy surface of iron leaving a nickel-rich oxide layer on the alloy surface which oxide layer is very hard to remove by pickling. With some alloys, scale tends to adhere to more strongly or penetrate more deeply at certain locations as opposed to others. With such alloys, while the actual scale loss may not be excessive, additional losses are involved in the labor required to scarf or remove local patches of scale. If, as occasionally happens, a semi-worked billet or slab is not thoroughly scarfed, the remaining scale patches become areas of defects in the product of further working. Although most scale, when cool, is brittle, hot scale is often sufficiently workable so that a minor scale defeet on a slab can become a large defect area on a hotrolled plate. With 9% nickel steel local penetration of oxides at grain boundaries is a severe problem.

The problems of scaling, carburization, decarburization and surface alloy enrichment (or depletion) are further magnified when the metal is not in dense, massive form but rather is in the form of a porous powder compact. In a porous powder compact, the surface of the metal extends into the innermost reaches of the body. Any undesirable reaction with an atmosphere occurs not only on the exterior surface which can be reached, but also on the interior surfaces which, after further compaction are completely inaccessible. Although in some instances a controlled amount of interior oxidation can be desirable and to avoid it, the powder metallurgy art customarily employs the step of inert atmosphere sintering of pressed powder compacts prior to hot working the compacts. Because a truly inert atmosphere sintering operation is costly, it would be highly advantageous to be able to sinter in atmosphere which are not truly inert, e.g., atmospheres having a relatively high dew point.

It has now been discovered that by means of a special process the surface of metals can be protected from interaction with the surrounding atmosphere especially at elevated temperatures and under conditions of mechanical deformation such as existing prior to and during hot rolling, hot forging and the like.

It is an object of the present invention to provide a novel process for protecting the surface of metals from interaction with surrounding atmospheres.

Another object of the present invention is to provide protected metal bodies specifically adapted to be exposed to oxidizing atmospheres at high temperatures. i.e., temperatures in excess of about 1000F. (about 500C.), and to be worked in said atmospheres at said high temperatures which protected metal bodies and the products of'working said protected metal bodies are only minimally deteriorated, if at all, by oxidation, decarburization and the like at said temperatures.

A further object of the present invention is to provide novel compositions of matter useful in the process of protecting the surface of metals from interaction with surrounding atmospheres.

Further objects and advantages will become apparent from the following descriptions.

Generally speaking, the present invention contemplates a process for protecting metal from the deleterious effects of interaction with atmospheres surrounding said metal which comprises forming on the surface of said metal a continuous film of chromium oxide which is green at room temperature (i.e., a green chromium oxide) and silica, said film directly overlying the underlying metal and said green chromium oxide coacting with the silica and any copresent alkali metal oxide to provide a film impervious to gas at temperatures of about 500C. to about 1300C. The invention contemplates the product so protected and a composi tion of matter comprising chromium metal powder dispersed in an alkali-stabilized silica sol particularly adapted to be used in forming the green chromium oxide-silica film on metals.

METALS WHICH CAN BE PROTECTED The present invention is particularly concerned with minimizing as far as possible scaling and decarburization of ferrous-base metals in oxidizing atmospheres. As used in the present specification and claims, the term ferrous-base metals includes iron-base alloys containing at least about 50% iron, up to 30% chromium and up to 30% of other alloying ingredients normally employed to provide steels of commerce. The process of the present invention is particularly useful in inhibiting or minimizing the scaling of ferrous-base alloys such as mild steel, 9% nickel steel and austenitic, ferritic and duplex structured stainless steels. The present process is also useful for inhibiting decarburization of tool steels and iron-base powder compacts containing free graphite.

While the ferrous-base alloys mentioned hereinbefore represent the greater bulk of metal production which can be facilitated by means of the present invention, the invention is also applicable to the protection of surfaces of other metals or alloys which are normally heat treated and worked in air and melt (i.e., have soliduses) at temperatures in excess of about l000C. and which contain from 0% to about 40% chromium. For example, the process of the present invention can be employed to prevent the formation of decarburized skins or carbon-depolarized nickel anode material and to inhibit oxidation of copper-base alloys, titaniumbase alloys, nickel-base alloys and cobalt-base alloys. As with stainless steels, the value of the coating of the present invention with chromium-containing, nickelbase, and cobalt-base alloys is not so much to prevent metal loss by sealing but rather to minimize the formation of thin, tightly adherent, difficultly removable oxide layers which often penetrate preferentially in grain boundary regions.

PROCESS Metal to be protected in accordance with the present invention, having a clean metallic surface, i.e., a surface devoid of any substantial amount of non-metallic substances such as oxides, grease, etc., is coated with metallic chromium and an aqueous alkali-stabilized silica sol both components being applied either simultaneously or the chromium coating being applied first, the total thickness being at least one micron. The coated surface is then dried in such a manner as to provide a continuous dry film on the metal surface and to avoid blistering and thereafter fired in an atmosphere at least oxidizing to chromium to produce a composite, continuous, green chromium oxide-silica film directly overlying the underlying metal surface. For all practical purposes when hot working steel products, the chromium metal-silica film on the original billet can be as thick as desired but usually it need be no thicker than about 25 to about 60 or possibly 100 microns. Using electrodeposits of chromium followed by a silica coating derived from a silica sol as described hereinafter satisfactory results have been achieved with coatings as thin as about 10 microns. Under ordinary conditions, film will persist on the metal surface during hot working and will provide some lubricating effect. Clearly, the film on the product of hot working will be considerably thinner than the original film, perhaps by an order of magnitude or greater. Further, after hot working, it will often be found that chromium is diffused into the surface of the underlying metal.

The amount of chromium which is required to be coated on the clean metal surface differs substantially with the method of application and with the nature of the base metal. When chromium is electroplated on the surface or deposited by chromizing techniques, a layer of about 2 X 10' inch (about 5 X cm.) (0.5 micron) on ferrous-base metal is all that is necessary. This thickness of chromium represents a dosage of about 3.6 grams of chromium per square meter of metal surface.

lt has been found that on low alloy steel which has been subsequently hot rolled, an electroplate of chromium 0.25 micron thick was not adequate for purposes of the invention. It is possible, however, that with alloys which contain substantial amounts of chromium within the range set forth hereinbefore a 0.25 micron thick electroplated chromium layer can sometimes be sufficient for purposes of the invention. With copper-base, nickel-base and cobalt-base alloys, it is necessary to use substantially more chromium than can conveniently be applied by electroplating.

Because electroplating and chromizing cannot always be conveniently carried out under prevailing industrial conditions and because of the chromium dosage requirements of alloys other than ferrous-base alloys, applicant also contemplates coating the metal to be protected with a layer of particulate chromium metal advantageously applied as a slurry in an aqueous, alkali-stabilized silica sol. The particles of chromium metal, when substantially equiaxed, advantageously have mean particle diameters in the range of about 0.001 micron to about 25 microns. With chromium particles of about 3 microns average dimension, it has been found that good results are obtained on steel to be hot worked with dosages of about to grams per square meter (g/m of metal surface area. Reasonably good protection for short heating cycles is obtained on steel billets with dosages as low as about 50 g/m of 3 microns sized chromium particles on the billet surface. This dosage is theoretically equivalent to a solid chromium layer about 7 microns thick. If desired, and especially when the metal is to be exposed to oxidizing atmospheres at high temperatures, e.g., about l250C. and higher, the dosage rates of chromium particles on the surface can be as high as about 500 grams per square meter.

Other means of providing a thin layer of chromium on the metal surface can also be used. For example, an iron-rich surface can be gas chromized with chromic chloride vapor. Chromium layers can be produced on any metal surface by thermal decomposition of chromium carbonyl and other thermally decomposable metallo-organic chromium compounds, such thermal decomposition processes being disclosed, for example, in U.S. Pat. No. 2,793,140 to Ostrofsky et al. and in U.S. Pat. No. 2,898,235 to Bulloff.

The silica of the protective green chromium oxidesilica film used in the present invention is advantageously derived from alkali-stabilized, aqueous silica sols marketed by El. duPont de Nemours and Company, Inc. (du Pont) under the trademark Ludox and other designations and made in accordance with the teachings of any one or more of the Bechtold et al U.S. Pat. No. 2,574,902, the Rule U.S. Pat. No. 2,577,485, the Alexander U.S. Pat. No. 2,750,345, the Bird U.S. Pat. No. 2,244,325, the Voorhees U.S. Pat. No. 2,457,971 and the ller U.S. Pat. No. 2,668,149. Other silica sols are also operative provided that they contain an effective amount of alkali metal, e.g., sodium, potassium, lithium ion associated with the negative charge sites on the silica sol particles. Generally the mole ratio of silica to alkali metal in such sols is about 4:1 to about 400:1 Operative silica sols also can be modified to contain oxidic aluminum in place of part of the oxidic silicon on the surface of the silica sol particle. Such an alumina-modified silica sol is sold by du Pont under the trade designation Ludox AM. While the aforedescribed alkali-stabilized silica sols are, at present, the best known means of providing the required silica in the green chromium oxide-silica films formed in situ on metal surfaces in accordance with the present invention, other means of forming silica films should not be overlooked. For example, silica films containing alkali metal ion can be obtained by alkali hydrolysis of ethyl silicate, by partial neutralization and dialysis of 1:1 sodium silicate (water-glass) solutions and other means well known to those skilled in the art. For purposes of the invention, however, it is necessary that at least about one mole of alkali metal ion associated with negative charge sites on the silica sol particle can be present for about every 400 moles of silica copresent. Dosage rates for the silica portion of the coating advantageously are of the order of 5 to 60 g/m with associated ion from the group of sodium, potassium and lithium ion, measured as the oxide, advantageously being present in an amount of 0.5 to 2.5 parts for every 10 parts by weight of silica. Advantageously, when used as a slurry the chromium, silica and alkali metal ion are in weight ratios within the ranges of 6 to I parts by weight of chromium to 0.4 to 2 parts by weight of silica to 0.02 to 0.2 parts by weight of alkali metal oxide.

To complete the formation of the green chromium oxide-silica coating on the surface of the metal to be protected, the surface bearing the chromium, the silica and the alkali metal is exposed to an atmosphere containing oxygen or an oxygen donor which atmosphere is at least oxidizing to chromium. The surface should be held at about 1500F. (about 800C.), to about I700F. (about 930C.) until the continuous green chromium oxide-silica film is established. Merely heating the object to be protected in furnace air up to a 1700F. (about 930C.) (or higher) furnace temperature will ordinarily be a satisfactory method of establishing the protective coating of the present invention. However, if unusually fast heating means such as skin-effect induction heating is used, it will be necessary to maintain the coated surface at or about I500F. (about 800C.) to about 1700F. (about 930C.) for some small amount of time, e.g., about 1 to about minutes, under oxidizing conditions in order to achieve the protective coating. Those skilled in the art will appreciate that metallic chromium will react with oxygen to form chromic oxide (Cr O and that this oxidation will produce about I33 kilocalories of heat per gram atomic weight of chromium. Thus, while the bulk of an object heated in a furnace to a furnace temperature of 1700F. (about 930C.) will have a maximum temperature of about I700F. (about 930C.) local micro-volumes within the chromium-silica coating will be at substantially higher temperatures. It is believed that the localized increments of temperature plus the volume expansion occurring upon oxidation of chromium cause mutual wetting of closely spaced green chromium oxide particles and silica and the formation of a continuous protective film on the metal surface.

The film formed at 800C. or higher protects the underlying metal from reaction with the atmosphere at high temperatures. It is to be understood, however, that in order for this film to be effective, it must directly overlie metal. Accordingly, it is necessary that the initially applied ingredients, i.e., chromium metal and silica form or be included in a film which will prevent oxidation of underlying metal prior to formation of the film of the present invention.

The amount of alkali metal ion present in the compositions useful in the practice of the present invention is limited to the aforestated maximum amounts in relation to the silica in order to avoid formation of fully liquid silica-alkali metal oxide phases at the elevated temperatures at which protection is required, e.g., up to about 2350F. (1290C.). Based upon tests made at 2300F. (I260C.) on carbon steel specimens coated with chromium-containing, silica-lithia coatings having a range of ratios of silica to lithia at dosages between about I60 g/m and about 230 g/m (coating weight per metal surface area) maximum metal protection appears to be obtained with ratios of about 5 to 1 to about 17 to I by weight of silica to lithia. Based upon these same tests, it appears advantageous, when maximum metal protection is required at high temperatures, to avoid weight ratios of about 2 to I silica to lithia and about 28 to I silica to lithia.

Slurry compositions containing chromium metal particles dispersed in an alkali-stabilized aqueous silica sol and usable in accordance with the present invention for coating on metal to be protected can contain about 5 to about 10 parts by weight of chromium, about 0.4 to about 2 parts by weight of sol form silica, about 0.02 to about 0.3 parts by weight of alkali metal ion (measured as Me O) the ratio of silica to Me O being at least about about 4, all components being dispersed or dissolved in about 4 to about 20 parts by weight of water.

Total solids weight of the coating per unit area of surface being protected is an important determinant ofthe protection providable by means of the slurry compositions of the present invention. Table I sets forth in terms of temperature, degrees of protection and solids dosage per unit area, typical results of protection obtainable with the slurry compositions of the present invention used on carbon steel which, after coating, was held at the indicated temperature for 1 hours.

The data set forth in Table I is pertinent to slurries made up with chromium powder of about 2 to about 3 microns average dimension and carefully fractionated to exclude particles outside this size range. When larger sized chromium particles are used, even in small amounts, in the slurry compositions of the invention, it is usually necessary to use much heavier layers of coating to achieve the same protection obtainable with lighter coatings of the slurries containing finer chromium particles.

Those skilled in the art will appreciate that watersoluble polar liquids such as alcohols, glycols, etc., can be substituted for some or all of the water in the slurry compositions of the present invention. However, in general, such substitution merely adds to cost and hazard without producing any particular advantages. Water-soluble or water-dispersible resins and other polymeric substances can sometimes be included with advantage in the chromium-containing slurry compositions of the present invention in order to increase viscosity, enhance the film forming of the silica sols and for other purposes. Some resins and polymeric substances compatible in the slurry compositions of the present invention are set forth in Table II. The list of Table II is given, however, with the caution that the materials listed therein should be used in small proportions, if at all, in order to avoid interference with the mechanisms involved in forming directly on underlying metal the continuous, green chromium oxide-silica film as described hereinbefore.

TABLE II Acrylic Co-polymer Emulsions Acrylic Emulsions -e.g., Neo-cryl A. 234.U or Rhoplex D -e.g., RWU-20l or Rhoplex 360A TABLE II (ontinued RESINOUS POLYMERIC MATERIALS COMPATIBLE IN THE SLURRY COMPOSITIONS OF THE INVENTION e.g., Carbowax 4000 Ammonia Cut Ammonia Cut Dexlrins Gelatins Alginates Gum Arabic Gum Tragacanth Carob Gum Mucilage Starch Corn Syrups Xanthan Gum Shellac Ammonia cut Polyvinylpyrrolidone -e.g,, Neo-Rex' 5-71 or RWL 110 e.g,, Durez 17211 or Durex 19788 Na Carboxymethyl Cellulose Alkyl Hydroxyalkyl Celluloses Polyacryllic Acid Ammonia Cut Trademark of Polyvinyl Chemicals. Inc. "Trademark of Morton Chemical Co. "Product Designation of Morton Chemical Co. Trademark of U.B.S Chemical Cor Trademark of Hooker Chemical Corp.

Trademark of Union Carbide Corp.

Those skilled in the art will appreciate that many of the natural products named in Table II which can be used as thickeners are subject to bacterial or enzyme deterioration. In order to inhibit deterioration, preservatives such as sodium phenate, sodium chlorophenate or other well-known bacteriocides or enzyme inactivators can also be included in the slurry compositions of the present invention in conventional amounts, e.g., 0.1%, based on weight of material being protected.

In order to maximize the formation of continuous films or metal surfaces, the chromium-containing slurry compositions of the present invention as well as the silica sols used with electroplated or chromized chromium layers advantageously contain either anionic or nonionic surfactants. Anionic surfactants such as sodium alkyl aryl sulfonate and sodium lauryl sulfate in amounts of 0.1% to 0.3% by weight based upon total silica plus alkali metal ion plus water are generally useful in the practice of the present invention. Nonionic surfactants such as ethylene oxide condensates, octylphenoxydiethoxyethanol, alkylphenoxypoly(ethyleneoxy)ethanol, polyethylene glycol tertiary-dodecylthioether and trimethylnonanol can also be used especially in conjunction with alkali-stabilized silica sols having a SiO /Me O ratio below about 120.

It is within the contemplation of the invention to include in the chromium-containing slurry compositions along with silica and the alkali metal ion, small amounts of other ingredients such as alumina, magnesia, lime, clay (e.g., bentonite), and like materials which can alter somewhat the character of the green chromium oxide-silica film on the protected metal surface. As a specific example, inclusion of aluminum in an amount of up to about 60% (by weight of the chromium metal present) in the slurry compositions of the present invention promotes pop-off of the film upon cooling after the steel has been heat treated. Table III sets forth (in percent by weight of chromium metal) amounts of these other ingredients which can be used in the slurry compositions of the present invention and which, in contrast to the organic additives set forth hereinbefore, do not burn off but rather become part of the fully oxidized film.

TABLE III 7: by weight of Ingredient copresent chromium A1 0 10 Ni 200 Al 60 Bentonite 2 Rare earth metal oxide 2 Those skilled in the art will appreciate that metals such as aluminum cannot be copresent with relatively highly alkaline silica sols (e.g., sols having a SiO /Me O ratio of less than 20) for any extended time without causing hydrogen evolution and sol gelation. Accordingly, if one desires to use aluminum in a slurry composition made with a highly alkaline silica sol, the aluminum should be added just prior to coating the slurry onto a metal surface. Use of rare earth metal oxide, e.g., yttrium oxide, is particularly advantageous when nickel and aluminum together are used to dilute the amount of chromium in the slurry compositions of the present invention.

In order to give those skilled in the art a better understanding and a greater appreciation of the invention, the following examples are given.

EXAMPLE 1 A clean piece of SAE 1020 carbon steel (1 inch X 1 inch X 6 inches) (2.5 X 2.5 X 15.25 cm.) was coated over part of its surface by first, electroplating part of the surface with 0.00002 inch (about 0.00005 cm.) thickness of chromium from a standard chromic acidsulfate bath, coating the plated portion of the surface with alkali-stabilized silica sol, allowing the sol to dry to a layer less than 1000 angstroms thick and heating the thus partially coated metal. The heating was in air up to 2000F. (about 1090C.) at which temperature the metal was held for 1% hours. The metal piece was then hot rolled to /a-inch (0.32 cm.) thickness. The non-treated portion had heavy loose scale as expected; whereas a very thin, tenacious green chromium oxidesilica coating was observed on the treated portion of the specimen. Metallographic examination revealed no decarburization under the protective layer and no sealing appeared on the protected surface. The alkali stabilized silica sol used in treating part of the surface of the metal piece had a SiO /Me O weight ratio of 50.

EXAMPLE ll Specimens of Type 430 ferritic stainless steel and a duplex structured stainless steel were coated as was the coated portion of the steel piece in Example 1. Similar specimens were given only the chromium plating treatment. Under oxidation in air at 1800F. (about 980C.) the specimens which were only chromium plated received no significant protection. Both of the specimens treated in accordance with the present invention had good oxidation resistance for 6 hours at 1800F.

EXAMPLE III.

A 30 pound ingot of M tool steel (0.85% C, 4% Cr, 6.4% W, 5% Mo, balance essentially Fe) was cast and forged to a 2 /2 inch (6.35 cm.) bar. A piece of this bar was coated as described in Example I and heated to 2075F. (about 1140C.) in air for 30 minutes. The coating protected the surface from oxidation and give virtually complete resistance to decarburization.

EXAMPLE IV A 9% nickel Steel (0.1% C, 9.0% Ni, 0.4% Mn, 0.3% Si, 0.015% P, 0.015% S, balance essentially Fe) in the form of a l-inch (about 2.5 cm.) diameter bar was partially coated with the green chromium oxide-silica film as described in Example I. The specimen was then soaked in air at 2100F. (about 1150C.) for 10 minutes and then air cooled. A loose scale formed on the uncoated portion of the specimen and, after it flaked off, a tenacious nickel-rich oxide layer remained together with underlying grain boundary oxidation. In contrast, where the specimen had been coated, only slight oxidation resulted. The thin, protective green chromium oxide-silica film remained intact with no underlying nickel-rich scale or grain boundary penetra tion.

EXAMPLE V An Armco iron specimen of very low carbon content was coated with the green chromium oxide-silica film as described in Example 1 and thereafter pack carburized at 1700F. (about 930C.) for 2 hours along with an uncoated specimen. After the treatment, both specimens were sectioned and examined metallographically. The uncoated specimen was carburized deeply; whereas, the coated specimen was not carburized at all.

It is to be observed that the coating used on the protected specimen in Example V was originally heated in air to a temperature of about 1700F. (about 930C.) Without this original heating in air, no green chromium oxide-silica film is formed and substantially no protection of the metal surface is obtained.

EXAMPLE Vl A clean sample of SAE 1020 steel was coated at a dosage of about 0.1 gram/in (155 g/m with the dried residue of an aqueous slurry containing about 3.5 parts by weight of round grain chromium powder, the particles of which were about 3 microns in diameter. about 2.75 parts by weight of an aqueous lithium-stabilized 5 silica sol sold by E. 1. du Pont de Nemours and Company, Inc. as polysilicate 48 and containing about 20% by weight of SiO and about 2.1% by weight Li O with the balance being water and a volume of water equal to the volume of the silica sol. The coated sample was 1() then heated in air to about 1700F. (about 930C.) to form a continuous green chromium oxide-silica film on the steel surface. The steel was then capable of resisting high temperature oxidation both prior to and during hot working in the same manner as the coated portion of the steel sample described in Example 1.

To demonstrate that in situ oxidation of chromium metal particles is necessary for purposes of the invention, a slurry similar to theone used in Example VI was made up with green Cr O rather than with chromium metal. This slurry was applied to a steelsurface in the manner identical to that described in Example V1 to provide an equivalent (chromium metal) dosage and then heated in air to 1700F. (about 930C.) The coating thus formed gave substantially no protection against scaling at hot working temperatures. i.e.. temperatures well above 2000F. Use of the dried silica sol alone gives marginal protection only and only up to about 1500F. (about 815C.)

EXAMPLE Vll A portion of a specimen of a 9% nickel steel similar to that steel described in Example [V was coated on a machine ground surface with a slurry containing about 8 parts by weight of 3 micron chromium powder, about 5.85 parts of the lithium-stabilized silica sol described in Example V1 and about 5 parts by weight of water. The dried coating weighed approximately 0.094 g/in (146 g/m The thus coated specimen was heated in air to 2100F. (about 1150C.), soaked 4 hours at this temperature and hot rolled from 1 3/16 inches (about 3.0 cm.) to t-inch (about 0.65 cm.) in 8 passes.

The portion of the specimen having the green chromium-silica coating was completely protected whereas the non-coated portion of the specimen scaled heavily. Metallographic examination indicated a diffusion of chromium into the steel surface for a distance of about 25 microinches (about 0.62 micron). The surface of the rolled product still contained the green chromium oxide-silica coating on that portion corresponding to the originally coated portion of the specimen. This green coating can be removed by pickling, for example, in 15% by weight sulfuric acid, dried at C., or by sandblasting or by other methods well known to those skilled in the art.

EXAMPLES VlllXlV compositions in accordance with the present invention are set forth in Table IV.

TABLE [V Example No. V11 IX X X1 X11 X11] XIV XV Cr (pbw*) 8 5 5 4 5 5 5 3 Size of Cr particle (micron) 3 3 3 3 3 3 3 3 Al (pbw) l 3 1 l l TABLE IV Continued Example No. Vll IX X X1 X11 X111 XIV XV 501 (pbw) 3.5 5.85 5.85 11.7 5.85 5.85 5.85 5.85 Alkali metal Na Li Li Li Li Li Li Li Me O (pbw) 0.021 0.12 0.12 0.25 0.12 0.12 0.12 0.12 SiO (pbw) 1.05 1.17 1.17 2.34 1.17 1.17 1.17 1.17 Dilution water (pbw) 7.1 5 5 l 5 5 5 YgOg 0.1 0.1 Ni 5 'pbw parts by weight EXAMPLE XVI TABLE V (ontinucd C N 30 29, 00 2 1.5 Two specimens of carbon-depolarized nickel electro- D i 30 T plating anode material, one coated with a dried layer of the slurry of Example Vlll were soaked in air for 2 hours at 2l00F. (about 1 150C.) and then hot rolled from 1 3/16 inch (about 3.0 cm.) diameter bar to /8-111Cl'1 (about 1.6 cm.) diameter bar. The surfaces of both hot rolled samples were cleaned by anodic pickling in hydrochloric acid (1 to l dilution with water of commercial concentrated HCl). Metallographic exami nation of the specimen which was coated showed no decarburized surface layer whereas the uncoated specimen was significantly decarburized at or near the surface. Subsequent testing of the specimen which had been coated as an anode in a nickel electroplating bath indicated that this specimen could be used as a depolarized anode without a preliminary deskinning operation in contrast to the specimen which was rolled without the coating of the present invention and which required deskinning.

EXAMPLE XVll Four identical batches of mixed powders containing in percent by weight about 0.5% carbon (as graphite),

about 0.5% each of nickel and molybdenum, about U.T.S. Ultimate Tensile Strength ofsamples which were air cooled after heating at l700F. with no further heat treatment or mechanical working The data in Table V shows that the uncoated specimens were severely degraded by exposure to air at l700F. (about 930C.) whereas the coated specimens were virtually unaffected, exhibiting ultimate tensile strengths corresponding to similar material heated in a protective atmosphere. The results shown in Table V are important insofar as they show that by means of the present invention one can heat powder compacts in air prior to forging and not be forced to use cumbersome means to heat in protective atmosphere.

EXAMPLE XVlll Other tests carried out on powder masses containing in percent by weight 1% manganese, 4% nickel, 1% molybdenum, 0.5% carbon (as graphite) with the balance being essentially iron and compacted and sintered as described in Example XVll show that coating the compacts with chromium slurried in water or chromium slurried in alkali-stabilized silica sol will prevent oxidation of alloying elements during sintering in commercially available endothermic atmospheres, e.g., cracked ammonia of high dew point. After sintering, it is necessary, in order to obtain resistance to oxidation by formation of the green chromium oxide-silica film of the present invention, to recoat the sintered compact with alkali-stabilized silica sol. Specimen treatment and results pertinent to these other tests are set forth in slurry in an aqueous silica sol in accordance with the Table VI.

" TABLE VI Pre-Sinter PostSinter Time at U.T.S.* Specimen Coating Coating l700F. psi kg/mm E None None 0 79,800 56.1 F Cr None 0 95,000 66.7 G (r+A.S. None 0 96,600 67.9

siol ki i H None None 30 min. 47.000 33.0 E Cr+A.S. SiO, None 30 min. 52,000 36.6 .1 Cr+A.S. SiO A.S. SiO 30 min. 93.600 658 Ultimate Tensile Strength ofsamples air cooled with no further heat treatment or mechanical working Chromium alone ""Chromium Alkali-stabilized silica present invention and allowed to dry. All four compacts were heated in air at l700F. (about 930C.) as

specified in Table V, which table also contains the results of a room temperature ultimate tensile test conducted upon the sintered and reheated compacts.

The data in Table V1 as to specimens E to G shows that chromium alone is sufficient to protect a powder compact from deleterious oxidation in a cracked ammonia atmosphere of high dew point. However, when subsequent heating in air is contemplated, it is necessary, as shown by the data as to samples H, 1 and J, to recoat the compact with silica if the original coating was heated in a reducing atmosphere. While the reason for this is not fully known, it is possible that under reducing conditions in association with a porous metallic member, molten material rich in alkali metal oxide may be abstracted from the coating by capillary action or otherwise.

EXAMPLE XIX 1500F. (about 815C.) to 2350F. (about 1300C.), the thin (i.e., up to about 100 microns thick) green chromium oxide-silica coatings of the present invention can also find use in protecting steel and other metals A Sample Of a chromium-Containing cupro-niokel from atmospheric corrosion at any temperature. Thus, all y having a dus f about l 0C- and a n ng while it may at times be advantageous to include alumi- Percent y Weight about 30% nickel, about 28% num in the coatings of the present invention to provide Chromium, ab011t0-7 manganese, about 015% Zircofor flake-off of the coating upon cooling after hot worknium, about 0.05% titanium, with the balance being esing, at other times it is advantageous to employ a coatsentially copper was coated with a chromium-alkali- 10 ing of the invention which will be adherent to the stabilized silica sol slurry of the present invention and cooled metal and thus help protect it from rusting was heated in air, along with an uncoated sample, at under ambient storage conditions. The coatings of the about 1020C. for 2 hours. The uncoated sample had invention adherent to cooled, hot worked steels and a substantial metal loss whereas the coated sample had other metals are also highly useful as bases for organic no loss. The uncoated sample had a difficultly removand inorganic protective coatings such as paints, lacable oxide layer whereas the coated sample was easily quers, bonded polymeric films, etc. cleanable. Microprobe analysis results of metal near The coatings of the present invention are distinctly the surface is set forth in Table VII, in which Table eledifferent, both in comp n n in effectiveness, mental content is in weight percent with oxygen being from silica coatings of the prior art such as disclosed in l l d b diff n e the Cupery et al. US. Pat. No. 3,133,829. Samples of TABLE VII Type 304L stainless steel were treated as follows: Sample l was left uncoated as a control; Sample 11 was coated with a film derived from an alkali stabilized sil- Sample Dlszlilr paecgrom (g; 5;; 3 ,2 ica sol; and Sample III was coated with a composition (micron) of the present invention including chromium metal dispersed in an alkali-stabilized silica sol (coating weight 8223323 ,2 12 23;? Q2 f 240 g/m All the samples were heated at 2200F. Coated 6 31.9 67.1 2. 0.0 (about 1200C.) in air for 4 hours, cooled and pickled in an aqueous nitric acid-hydrofluoric acid pickling 30 bath to bare metal. The control (Sample I) lost weight The data of TAble Vll shows the complete effectivein an amount of abOut2l86 g/m2 of Surface area Sample ness of the coating of the present invention in eliminat- H l about 665 of Surface area and Sample ing deterioration of the aforedescribed copper-base gamed about 12 g/m of Surface area The i alloy by high temperature Oxidation tshes of Samples 1 and II were badly deteriorated From the foregoing those skilled in the art will whereas the surface f1n1sh of Sample Ill was barely afpreciate that the present invention is broadly applicafected by the heamig and plcklmg i i data ble to the protection of metals of the kind described show that films der1ved from alkal1-stab1l1zed s1l1ca sol hereinbefore from deleterious or undesirable reaction m the absence of chromlum P f not protecnve with the atmosphere surrounding them. It is to be un to 9 mckel'comammg Stainless Steel at derstood that while best results are attained with the 40 but appear to demmfimalli/ affect slurry aspect of the present invention using pu or the ability of the stamless steel to resist oxrdatron. Sentially pure (unalloyed) chromium powder, useful Under the same conditions, the coatmg of the present results can also be attained through the use in slurry mvemlon Completely protectwe' form of metal owder containin reater than about chromium, e.g., various grad zs of ferro-chromium EXAMPLES XX TO along with the alkali-stabilized silica sols and with or Further examples of slurry compositions of the preswithout additions of chromium powder. When ferroent invention are set forth in Table VIII.

TABLE Vlll Example No. XX XXl xx11 XXlll Cr(pbw) 13 11.5 11.5 11.5 Size ofCr particle (microns) 3 3 3 3 Sol (pbw) 4.l 4.] 4.l 4 2 Alkali metal Li Li Li Li Me,o (pbw) 0.086 0.086 0.086 0.176 sio (pbw) 0.82 0.82 0.82 0.82 Dilution Water (pbw) 3.5 3.5 3.5 3.5 Carbopob" 941 0 03 0.03 0.03 0.03 CaO 0.1

Dry acid-base. water soluble copolym er sold by B. F. Goodrich Chemical Co.

chrome is used, the iron copresent with chromium can alter the color of the coating from green to various shades of brown without entirely eliminating the effect of the chromium oxide present in the coating. Further, if will be appreciated that although the foregoing description of the present invention emphasizes the protection of metals at elevated temperatures, e.g., about The composition of Example XXlll when applied on a carbon steel surface at a dosage rate of 180 g/m limited weight loss from solid steel due to 2 hours exposure to oxidizing atmosphere at 1260C. to about g/m Unprotected carbon steel subject to such exposure can be expected to exhibit metal weight losses due to scale formation of about 2000 to 3000 g/m It is to be understood that in order to achieve the most advantage from the slurry compositions of the present invention, it is necessary to provide uniform continuous coatings of dry slurry solids on the metal to be protected. In this regard, it has been found advantageous to dilute commercially available, lithia-stabilized silica sol by weight of silica) one to one by volume with water and to use about 0.4% by weight acid-base water soluble polymer in this sol-water mixture to provide a vehicle for chromium metal particles. The thus formulated vehicle is capable of efficiently carrying chromium particles onto a metal surface and retaining them there in a uniform continuous layer.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

I claim:

1. A protective coating impervious to gas at high temperatures of about 500C. to about 1300C. on metal having a melting point above about 1000C. and subject to be exposed to said high temperatures below said.

melting point comprising a thin continuous layer overlying said metal of chromic oxide formed in situ by oxidation of metallic chromium in the presence of the dried residue of colloidally dispersed alkali stabilized sol form silica, said alkali stabilized silica sol having a mole ratio of silica to alkali metal oxide of about 4 to l to 400 to 1, said layer being directly bonded to said underlying metal, and said layer comprising an essentially impervious barrier to gases at said high temperatures.

2. A protective coating as in claim 1 wherein the chromic oxide is green and is the result of in situ oxidation of essentially unalloyed chromium.

3. A protective coating as in claim 1 wherein the underlying metal is selected from the group of iron,

nickel, cobalt, copper and iron-base, nickel-base, co-

balt-base, titanium-base and copper-base alloys.

4. A protective coating as in claim 1 wherein the underlying metal is an iron-base alloy.

5. A protective coating as in claim 1 which also contains in situ oxidation products of at least one member of the group of aluminum and nickel.

6. A protective coating as in claim 1 wherein the alkali metal oxide is selected from the'group of sodium oxide, potassium oxide and lithium oxide.

7. A protective coating as in claim 1 wherein the silica is present in an amount of about 5 to about 60 grams per square meter of surface of said metal.

8. A protective coating as in claim 1 wherein the alkali metal oxide is lithia and the weight ratio of silica to lithia is about 5 to about 17.

9. A protective coating as in claim 2 wherein the underlying metal is an iron-base alloy and the green chromic oxide is the product of in situ oxidation of a chro-- mium plate.

10. A protective coating as in claim 2 wherein the green chromic oxide is the product of in situ oxidation of substantially equiaxed chromium powder having an average particle size of about 0.001 to about 25 microns.

11. A protective coating as in claim 10 wherein the chromium powder oxidized in situ is associated with silica and alkali metal oxide in an amount of about 5 to about 10 parts by weight of chromium to about 0.4 to about 2 parts by weight of silica.

12. A protective coating as in claim 10 wherein chromium powder oxidized in situ is present in an amount of at least about 50 g/m of metal surface area.

13. A process for protecting the surface of metal melting above about 1000C. from the deleterious effects of reaction with gases in contact therewith comprising oxidizing metallic chromium on the surface of said metal at a temperature of at least about 800C. while said metallic chromium is in intimate association with a mixture of silica and alkali metal oxide with the mole ratio of silica to alkali metal oxide being about 4 to 1 to about 400 to l to provide an essentially continuous, gas impermeable film comprising said silica, said alkali metal oxide and the product of the oxidation of said metallic chromium directly bonded to said metal surface.

14. A process as in claim 13 wherein essentially unalloyed chromium is oxidized.

15. A process as in claim 13 wherein the metal is selected from the group of iron, nickel, cobalt, copper and iron-base, titanium-base, nickel-base, cobalt-base and copper-base alloys.

16. A process as in claim 13 wherein metallic chromium is deposited on the surface to be protected in the form of a slurry of fine particles in an alkali-stabilized silica sol solution.

17. A process as in claim 16 wherein silica is present on the surface of said metal in an amount of about 5 to about 60 g/m of metal surface area.

18. A process as in claim 14 wherein the metal surface to be protected is the surface of an iron-base alloy and the unalloyed chromium is a chromium plate.

19. A process as in claim 13 wherein the alkali metal oxide is selected from the group consisting of sodium oxide, lithium oxide and potassium oxide.

20. A process as in claim 16 wherein the slurry is an aqueous slurry and contains about 5 to about 10 parts by weight of substantially equiaxed chromium particles having an average particle size of about 0.001 to about 25 microns, about 0.4 to about 2 parts by weight of so] form silica and about 0.02 to about 0.3 parts by weight of alkali metal ion measured as Me O associated with said sol form silica.

21. A process as in claim 16 wherein the surface to be protected is coated with at least about 50 g/m of chromium particles.

22. A process as in claim 16 wherein the surface to be protected is coated with chromium particles in an amount of about 50 to about 500 g/m of metal surface.

23. A slurry composition comprising particulate chromium dispersed in a liquid vehicle containing a film-forming alkali-metal stabilized silica sol, said chromium being in the form of substantially equiaxed particles having an average particle dimension of about 0.001 to about 25 microns and said silica sol having a mole ratio of silica to alkali metal oxide within the range of about 4 to l to about 400 to l.

24. A slurry composition as in claim 23 wherein the liquid vehicle is aqueous.

25. A slurry composition as in claim 23 wherein the liquid vehicle contains a thickener in addition to the silica sol.

26. A slurry composition as in claim 23 which contains about to about parts by weight of chromium to each about 0.4 to about 2 parts by weight of silica.

27. A slurry composition as in claim 23 wherein the chromium particles are about 3 microns in average dimension.

28. A protective coating impervious to gas at high temperatures of about 500C. to about l300C. on metal having a melting point above about lO0OC. and subject to be exposed to said high temperatures below said melting point comprising a thin continuous layer overlying said metal of chromic oxide formed in situ by oxidation at a temperature of at least about 800C. of metallic chromium in the presence of the dried residue of colloidally dispersed alkali stabilized silica sol, said silica sol being characterized by having at least I mole of alkali metal ion associated with every 400 moles of silica, said layer being directly bonded to said underlying metal and said layer comprising an essentially im pervious barrier to gases at said high temperature 29. A protective coating as in claim 28 which contains any one or more of the in situ oxidation products of nickel or aluminum, alumina, magnesia, lime, clay and rare earth metal oxide.

30. A protective coating as in claim 28 wherein the chromic oxide is green and is the result of in situ oxidation of essentially unalloyed chromium. 

2. A protective coating as in claim 1 wherein the chromic oxide is green and is the result of in situ oxidation of essentially unalloyed chromium.
 3. A protective coating as in claim 1 wherein the underlying metal is selected from the group of iron, nickel, cobalt, copper and iron-base, nickel-base, cobalt-base, titanium-base and copper-base alloys.
 4. A protective coating as in claim 1 wherein the underlying metal is an iron-base alloy.
 5. A protective coating as in claim 1 which also contains in situ oxidation products of at least one member of the group of aluminum and nickel.
 6. A protective coating as in claim 1 wherein the alkali metal oxide is selected from the group of sodium oxide, potassium oxide and lithium oxide.
 7. A protective coating as in claim 1 wherein the silica is present in an amount of about 5 to about 60 grams per square meter of surface of said metal.
 8. A protective coating as in claim 1 wherein the alkali metal oxide is lithia and the weight ratio of silica to lithia is about 5 to about
 17. 9. A protective coating as in claim 2 wherein the underlying metal is an iron-base alloy and the green chromic oxide is the product of in situ oxidation of a chromium plate.
 10. A protective coating as in claim 2 wherein the green chromic oxide is the product of in situ oxidation of substantially equiaxed chromium powder having an average particle size of about 0.001 to about 25 microns.
 11. A protective coating as in claim 10 wherein the chromium powder oxidized in situ is associated with silica and alkali metal oxide in an amount of about 5 to about 10 parts by weight of chromium to about 0.4 to about 2 parts by weight of silica.
 12. A protective coating as in claim 10 wherein chromium powder oxidized in situ is present in an amount of at least about 50 g/m2 of metal surface area.
 13. A PROCESS FOR PROTECTING THE SURFACE OF METAL MELTING ABOVE ABOUT 1000*C. FROM THE DELETERIOUS EFFECTS OF REACTION WITH GASES IN CONTACT THEREWITH COMPRISING OXIDIZING METALLIC CHROMIUM ON THE SURFACE OF SAID METAL AT A TEMPERATURE OF AT LEAST ABOUT 800*C. WHILE SAID METALLIC CHROMIUM IS IN INTIMATE ASSOCIATION WITH A MIXTURE OF SILICA AND ALKALI METAL OXIDE WITH THE MOLE RATIO OF SILICIA TO ALKALI METAL OXIDE BEING ABOUT 4 TO 1 TO ABOUT 400 TO 1 TO PROVIDE AN ESSENTIALLY CONTINUOUS, GAS IMPERMEABLE FILM COMPOSITION COMPRISING PARTICULATE CHROMIUM DISPERSED IN A LIQUID VEHICLE CONTAINING A FILM-FORMING ALKALIMIUM DIRECTLY BONDED TO SAID METAL SURFACE.
 14. A process as in claim 13 wherein essentially uNalloyed chromium is oxidized.
 15. A process as in claim 13 wherein the metal is selected from the group of iron, nickel, cobalt, copper and iron-base, titanium-base, nickel-base, cobalt-base and copper-base alloys.
 16. A process as in claim 13 wherein metallic chromium is deposited on the surface to be protected in the form of a slurry of fine particles in an alkali-stabilized silica sol solution.
 17. A process as in claim 16 wherein silica is present on the surface of said metal in an amount of about 5 to about 60 g/m2 of metal surface area.
 18. A process as in claim 14 wherein the metal surface to be protected is the surface of an iron-base alloy and the unalloyed chromium is a chromium plate.
 19. A process as in claim 13 wherein the alkali metal oxide is selected from the group consisting of sodium oxide, lithium oxide and potassium oxide.
 20. A process as in claim 16 wherein the slurry is an aqueous slurry and contains about 5 to about 10 parts by weight of substantially equiaxed chromium particles having an average particle size of about 0.001 to about 25 microns, about 0.4 to about 2 parts by weight of sol form silica and about 0.02 to about 0.3 parts by weight of alkali metal ion measured as Me2O associated with said sol form silica.
 21. A process as in claim 16 wherein the surface to be protected is coated with at least about 50 g/m2 of chromium particles.
 22. A process as in claim 16 wherein the surface to be protected is coated with chromium particles in an amount of about 50 to about 500 g/m2 of metal surface.
 23. A SLURRY COMPOSITION COMPRISING PARTICULATE CHROMIUM DISPERSED IN A LIQUID VEHICLE CONTAINING A FILM-FORMING ALKALIMETAL STABLIZED SILICA SOL, SAID CHROMIUM BEING IN THE FORM OF SUBSTANTIALLY EQUIAXED PARTICLES HAVING AN AVERAGE PARTICLE DIMENSION OF ABOUT 0.001 TO ABOUT 25 MICRONS AND SAID SILICA SOL HAVING A MOLE RATIO OF SILICA TO ALKALI METAL MTAL OXIDE WITH RANGE OF ABOUT 4 TO 1 TO ABOUT 400 TO
 1. 24. A slurry composition as in claim 23 wherein the liquid vehicle is aqueous.
 25. A slurry composition as in claim 23 wherein the liquid vehicle contains a thickener in addition to the silica sol.
 26. A slurry composition as in claim 23 which contains about 5 to about 10 parts by weight of chromium to each about 0.4 to about 2 parts by weight of silica.
 27. A slurry composition as in claim 23 wherein the chromium particles are about 3 microns in average dimension.
 28. A protective coating impervious to gas at high temperatures of about 500*C. to about 1300*C. on metal having a melting point above about 1000*C. and subject to be exposed to said high temperatures below said melting point comprising a thin continuous layer overlying said metal of chromic oxide formed in situ by oxidation at a temperature of at least about 800*C. of metallic chromium in the presence of the dried residue of colloidally dispersed alkali stabilized silica sol, said silica sol being characterized by having at least 1 mole of alkali metal ion associated with every 400 moles of silica, said layer being directly bonded to said underlying metal and said layer comprising an essentially impervious barrier to gases at said high temperature.
 29. A protective coating as in claim 28 which contains any one or more of the in situ oxidation products of nickel or aluminum, alumina, magnesia, lime, clay and rare earth metal oxide.
 30. A protective coating as in claim 28 wherein the chromic oxide is green and is the result of in situ oxidation of essentially unalloyed chromium. 